The present invention relates to a bone screw for implants for fixation of bone fragments at fractures wherein the bone screw comprises a threaded or tapped part which is adapted to be screwed into a bone fragment at the fracture. The threaded part extends backwards from a front end portion of the bone screw to an untapped part thereof and has threads with the same outer diameter. The threads in the threaded part extend from a core and the threads of the threaded part have the same pitch of thread. The invention also relates to a method for producing the threads of the bone screw and a drill for drilling holes therefor.
Bone screws for bone implants (so called lag screws) for fixation of bone fragments at femoral fractures are previously known from e.g. U.S. Pat. No. 5,836,950. At such bone screws, the width of the grooves between the threads is the same along the entire part. This design is not optimal for stable anchoring to surrounding bone material, particularly not if said bone material is osteoporotic.
There are also prior art bone screws in which the threaded part has a cylindrical core. For being able to screw such a bone screw into bone material, one normally has to predrill said bone material for obtaining a hole therein for the cylindrical core or one has to use a special bone screw which at the front is provided with a drill bit for obtaining said hole.
At prior art bone screws, the threads are normally designed such that one has to cut threads in the bone material for the threads of the bone screw either by means of a separate thread tap or a thread tap which is built into the bone screw.
The object of the present invention has been to provide a bone screw with improved properties for engaging bone material particularly if this is osteoporotic and also in order to obviate predrilling of the bone material for the core if said bone material is osteoporotic. This is arrived at according to the invention by providing the bone screw with the characterizing features of subsequent claim 1.
The object of the invention is also a method for providing the threads of the bone screw and this method includes the characterizing measures of subsequent claim 17.
A further object is to provide a drill for use if one has to predrill the bone fragment for engagement by the bone screw. This is arrived at by providing the drill with the characterizing features of subsequent claim 18.
Since the core of the bone screw according to the characterizing features of claim 1 is conical, it can be screwed into spongy bone material without predrilling thereof for the core. Also, the conical core functions as a xe2x80x9cploughxe2x80x9d which presses the surrounding bone material in radially outwards direction, i.e. it compresses the bone material closest to the threaded part of the bone screw.
Since the bone screw according to said characterizing features of subsequent claim 1 further comprises a threaded part with threads having truncated crests, the width of which increases in backwards direction, i.e. with a thin thread at the front and thicker threads further backwards, it is possible to cut threads in the surrounding bone material directly with the threads of the bone screw instead of doing it with a separate thread tap or a thread tap built into the bone screw.
Since the width of the groves between the threads decreases because of the increase in width of the thread crests, it is achieved that the surrounding bone material is compressed by the threads when the bone screw is screwed into said bone material.
A further advantage with a conical core, that the threads are thin at the front and increase gradually in width and that the width of the grooves between the threads decreases, is that the bone screw gets a firm grip also in osteoporotic bone material when it is screwed into such bone material.